1. Field of the Invention
The present invention relates to the production of phase shifting masks for extreme ultraviolet lithography (EUVL), and more specifically, it relates to systems and methods for directly writing patterns into the reflective multilayer coating of an extreme ultraviolet lithography phase shifting mask and providing a patterned absorber layer onto the EUVL mask.
2. Description of Related Art
Phase shifting masks (also known as reticles) are commonly used as a resolution enhancement technique in optical lithography and the technology is well established and widely used in deep ultra-violet lithography systems. See U.S. Pat. No. 5,045,417, Okamoto et al., titled “Mask For Manufacturing Semiconductor Device And Method Of Manufacture Thereof” issued 1991. Current DUV masks are transmissive and are designed to alter both the phase and amplitude of the transmitted light. In particular, the alternating phase shifting mask (alt-PSM) has been developed to extend the resolution limit of DUV optical systems. The fundamental quantity of interest in determining lithographic resolution is the normalized image line slope (NILS) as this is what determines the sharpness of the lines that can be printed. A common factor used to estimate the smallest printable feature size is the k1 factor of the printing process. For a printing system of a given numerical aperture (NA) operating at a given wavelength (λ) the critical dimension (CD) is given by:   CD  =            k      1        ⁢                  λ        NA            .      
A lower CD means the ability to print smaller lines, and a smaller value of k1 means that smaller lines can be printed on the same optical system. The factor k1 is dependent on the design of the mask used. See U.S. Patent Application No. US2001/0021475, 2001, titled “Lithography Method And Lithography Mask” to Czech et al. For binary transmission masks, k1 lies in the range of 0.5 to 0.7 (the Rayleigh limit of resolution). Halftone masks enable k1 to be reduced to values of 0.38 to 0.55, whilst phase shift masks enable k1 to lie in the range of 0.2 to 0.38.
Phase shift masks can improve the CD specification in a number of ways. These include, but are not limited to:
1. Direct resolution enhancement By taking advantage of both intensity and phase modulation, it is possible to control more of the complex number space defining the optical wave-field leaving the mask, and therefore increase the information content of the light field. This can be used to directly reduce the k1 factor, thus, the printable feature size.
2. If k1 can be improved, it is possible to reduce the NA for a given CD specification. High NA optics are generally larger and harder to fabricate than low NA optics, thus improving k1 through phase shifting enables the specifications on the size and NA of the optics to be relaxed for a given CD specification, thereby reducing the cost of fabricating the optics set.
3. Maintaining the CD specification by reducing k1 and moving to a lower NA also increases the process window. Lower NA optics have a greater depth of focus, thus the focusing tolerances in the wafer plane are relaxed and it is possible to use more economical stages to scan the wafer.
4. Flare control: Flare in the wafer plane can vary as the mask is scanned due to variations in feature density on the mask, with the flare variation affecting contrast in the image plane and, thus, minimum feature resolution. The addition of superfluous phase and amplitude features to the mask can be used to control this.
Although the concept of a phase shifting mask can be directly extended to EUV lithography, the existing technologies for making DUV phase shifting masks are not compatible with EUV mask technology. An EUV mask consists of a thick opaque substrate coated with a reflective multilayer film, on top of which is deposited an absorption layer. The absorption layer is patterned to produce regions that either allow or block reflections from the underlying multilayer coating. This is fundamentally different from the transmissive masks currently used in DUV lithography, and the technology for producing phase shifts in the DUV masks cannot be directly applied to reflective EUV masks.
The development of a new technology for the production of EUV phase shifting masks, such as the technique described in this ROI, would enable direct application of these existing image enhancement techniques to EUV lithography and would find direct application to the printing of smaller CD features using EUV optical systems.
Existing Technologies
With the potential need for phase shifting masks in EUV lithography, several strategies have already been developed for the production of EUV phase shifting masks. These basically fall into three categories: (a) introducing thickness variations by patterning the substrate prior to multilayer coating; (b) depositing phase shifting material on top of the multilayer during the patterning process, and (c) and etching the multilayer to introduce a refractive phase shift into the reflected light.
The first, illustrated in FIG. 1, involves patterning the substrate 10 prior to multilayer coating 12 with an additional layer 14 of well-controlled thickness. Patrick Naulleau et al, LBL, personal communication, October 2001. The multilayer deposited on this raised region of the substrate will reflect at a different phase as compared to the multilayers on the substrate itself, thereby forming a phase shift between the two parts of the pattern. A major drawback of this technique is that it requires patterning of the mask blank prior to deposition of the reflective multilayer, which would require the multilayer coating infrastructure to be incorporated into the patterning process line. This is at odds with the aim of semiconductor manufacturers to source unpatterned, multilayer coated mask blanks form external vendors. Furthermore, the smoothing process which takes place during multilayer deposition, and which is used to reduce the printability of substrate defects, would make the manufacture of sharp phase gradients and phase discontinuities difficult.
The second technique, illustrated in FIG. 2, involves depositing additional material 20, in addition to the absorber material 24 on top of the multilayer stack 22 (on substrate 26) to impart a refractive phase shift on the reflected light. See, for example, Czech et al., “Lithography method and lithography mask”, U.S. Patent Application No. US2001/0021475, 2001, p.2. This is analogous to the addition of more glass in transmissive optics to impart a phase shift into the transmitted light. The problem is that at EUV wavelengths the optical constants are not as forgiving as for visible light For example, a 43 nm thick layer of Molybdenum will impart a π phase shift on reflected 13.4 nm light, but will also reduce the reflected intensity by a factor of 0.6. Such localized loss of intensity associated with the production of the phase shift is not good when the aim is to produce a purely phase shifting mask, and could cause serious limitations on the practical utility of this approach to phase shifting.
A third technique, illustrated in FIG. 3, involves thinning (30) the multilayer 32 (on substrate 34) in order to impart the desired phase shift The multilayer stack also includes an absorber layer 36. This technique draws on ideas developed for the repair of amplitude defects in which it was shown that milling out craters in a reflective EUV multilayer imparts a refractive phase shift on the reflected light given by 2(2π/λ(1−n)h(x), where h(x) is the depth profile of the thinned out region, n is the refractive index difference between the media and the additional factor of two is due to reflection causing the light to pass twice through the region of refractive index change. Note that this phase shift is not the same as the phase shift imparted by reflection from the surface of the lowered profile, which would be given by 4πh(x)/λ. This is because the interference properties of the multilayer mirror force all wavefronts within the multilayer to be in phase, thus the phase shift imparted on reflection is due solely to refractive effects and not reflection from the top layer of the thinned region. For Mo/Si multilayers, (1−n)=0.03, so for normal incidence it would be necessary to remove 15 bilayers of material to achieve a phase shift of π in the reflected light. If there are sufficient bilayers in the multilayer before thinning takes place, this reduction in the number of layers will have little effect on the reflected intensity (subject of course to the terminating material, for example Mo rather than Si, not having absorption properties of its own). However there are significant problems with this idea:
1. The phase shifting features would be high aspect ratio trenches. Take the case of dense 1:1 features having a minimum feature size of 20 nm at the wafer. For a 4× magnification optical system the feature size on the mask would be 80 nm. The phase shifting trenches would be etched between every other feature, and would need to be less than 80 nm wide and 100 nm deep. This would be difficult to achieve cleanly in the multilayer without damaging the layers or getting undesired edge effects from the finite resolution of the ion mill.
2. The interaction of the radiation with the phase shifting features would be complicated, and would certainly include some diffraction from the side walls. This could lead to undesirable modulations of the aerial image that would be difficult to control.